This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
Polarization-division-multiplexed (PDM) quadrature duobinary (QDB) modulation with digital coherent detection has promise for use in high speed (e.g., 100 Gb/s or higher) optical communication. One problem with the coherent detection of QDB modulated signals is that cycle-slip can cause catastrophic error propagation. As understood by those skilled in the art cycle-slip can occur when the phase of the optical carrier output by a laser, e.g., a continuous sinusoidal wave signal, wanders.
Efforts to reduce error propagation in coherent detection when using PDM QDB modulation have attempted to recover the absolute carrier phase using periodically inserted pilot symbols in the transmitted data. Recovering the absolute carrier phase in this manner, however, can require more complex carrier phase recovery units and can reduce the spectral efficiency due to the insertion of pilot symbols into the transmitted data stream.